Compliment is to say something nice. Complement is to add to, enhance, improve, complete, or bring close to perfection. So, I can compliment your staff and their service, but if you have no current openings you have a full complement of staff. And your new app may complement your website.

For which I may decide to compliment you.

Criteria and criterion

“We made the decision based on one overriding criteria,” sounds pretty impressive but is wrong.

Remember: one criterion, two or more criteria. Although you could always use “reason” or “factors” and not worry about getting it wrong.

Discreet and discrete

Discreet means careful, cautious, showing good judgment; “We made discreet inquiries to determine whether the founder was interested in selling her company.”

Discrete means individual, separate, or distinct; “We analyzed data from a number of discrete market segments to determine overall pricing levels.” And if you get confused, remember you don’t use “discreetion” to work through sensitive issues; you exercise discretion.